kbuild: add support for ensuring headers are self-contained

Sometimes it's useful to be able to explicitly ensure certain headers
remain self-contained, i.e. that they are compilable as standalone
units, by including and/or forward declaring everything they depend on.

Add special target header-test-y where individual Makefiles can add
headers to be tested if CONFIG_HEADER_TEST is enabled. This will
generate a dummy C file per header that gets built as part of extra-y.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

.gitignore

@@ -22,6 +22,7 @@
 *.elf
 *.gcno
 *.gz
+*.hdrtest.c
 *.i
 *.ko
 *.lex.c

Documentation/dontdiff

@@ -19,6 +19,7 @@
 *.grep
 *.grp
 *.gz
+*.hdrtest.c
 *.html
 *.i
 *.jpeg

Documentation/kbuild/makefiles.txt

@@ -1018,6 +1018,13 @@ When kbuild executes, the following steps are followed (roughly):
 	In this example, extra-y is used to list object files that
 	shall be built, but shall not be linked as part of built-in.a.
 
+    header-test-y
+
+	header-test-y specifies headers (*.h) in the current directory that
+	should be compile tested to ensure they are self-contained,
+	i.e. compilable as standalone units. If CONFIG_HEADER_TEST is enabled,
+	this autogenerates dummy sources to include the headers, and builds them
+	as part of extra-y.
 
 --- 6.7 Commands useful for building a boot image
 

Makefile

@@ -1647,6 +1647,7 @@ clean: $(clean-dirs)
 		-o -name '*.dwo' -o -name '*.lst' \
 		-o -name '*.su'  \
 		-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \
+		-o -name '*.hdrtest.c' \
 		-o -name '*.lex.c' -o -name '*.tab.[ch]' \
 		-o -name '*.asn1.[ch]' \
 		-o -name '*.symtypes' -o -name 'modules.order' \

drivers/gpu/drm/i915/.gitignore

-header_test_*.c

drivers/gpu/drm/i915/Makefile.header-test

@@ -2,7 +2,7 @@
 # Copyright © 2019 Intel Corporation
 
 # Test the headers are compilable as standalone units
-header_test := \
+header-test-$(CONFIG_DRM_I915_WERROR) := \
 	i915_active_types.h \
 	i915_gem_context_types.h \
 	i915_priolist_types.h \
@@ -35,13 +35,3 @@ header_test := \
 	intel_sprite.h \
 	intel_tv.h \
 	intel_workarounds_types.h
-
-quiet_cmd_header_test = HDRTEST $@
-      cmd_header_test = echo "\#include \"$(<F)\"" > $@
-
-header_test_%.c: %.h
-	$(call cmd,header_test)
-
-i915-$(CONFIG_DRM_I915_WERROR) += $(foreach h,$(header_test),$(patsubst %.h,header_test_%.o,$(h)))
-
-clean-files += $(foreach h,$(header_test),$(patsubst %.h,header_test_%.c,$(h)))

init/Kconfig

@@ -96,6 +96,15 @@ config COMPILE_TEST
 	  here. If you are a user/distributor, say N here to exclude useless
 	  drivers to be distributed.
 
+config HEADER_TEST
+	bool "Compile test headers that should be standalone compilable"
+	help
+	  Compile test headers listed in header-test-y target to ensure they are
+	  self-contained, i.e. compilable as standalone units.
+
+	  If you are a developer or tester and want to ensure the requested
+	  headers are self-contained, say Y here. Otherwise, choose N.
+
 config LOCALVERSION
 	string "Local version - append to kernel release"
 	help

scripts/Makefile.build

@@ -294,6 +294,15 @@ quiet_cmd_cc_lst_c = MKLST   $@
 $(obj)/%.lst: $(src)/%.c FORCE
 	$(call if_changed_dep,cc_lst_c)
 
+# Dummy C sources for header test (header-test-y target)
+# ---------------------------------------------------------------------------
+
+quiet_cmd_header_test = HDRTEST $@
+      cmd_header_test = echo "\#include \"$*.h\"" > $@
+
+$(obj)/%.hdrtest.c:
+	$(call cmd,header_test)
+
 # Compile assembler sources (.S)
 # ---------------------------------------------------------------------------
 

scripts/Makefile.lib

@@ -66,6 +66,9 @@ extra-y += $(patsubst %.dtb,%.dt.yaml, $(dtb-y))
 extra-$(CONFIG_OF_ALL_DTBS) += $(patsubst %.dtb,%.dt.yaml, $(dtb-))
 endif
 
+# Test self-contained headers
+extra-$(CONFIG_HEADER_TEST) += $(patsubst %.h,%.hdrtest.o,$(header-test-y))
+
 # Add subdir path
 
 extra-y		:= $(addprefix $(obj)/,$(extra-y))